Field of the Invention
This invention relates to a wind turbine blade having a spar and shell made from a filament reinforced matrix in which the spar is mounted to the hub via a novel blade retaining structure having redundant load paths which provide a highly reliable joint.
Various retention devices for rotor blades such as aeronautical propellers are known, and include mechanisms such as flanges, thrust rings, thrust bearings, locking rings and split collars. Each of these devices require some type of modification to the shank of the blade such as the machining of races or the forming of raised surfaces thereon. Furthermore, in many of these prior art retention devices the shank portion is integral with a blade pitch adjusting mechanism, and is structurally adapted to absorb and transmit both centrifugal and transverse loads from the blade and the pitch change actuators.
In the development of large rotor blades such as those for wind-driven turbines or windmills, where the rotors are often 100 to 200 feet in diameter, the blades require special fabrication techniques to reduce weight and cost. Furthermore, the large rotating blades impose severe loads on the retaining devices, and improvements in the construction of such retaining devices is desirable to insure proper distribution of the loads.
One technique for reducing blade weight and cost without compromising structural rigidity involves the use of composite materials to form the blade. A filament reinforced matrix composite blade may be produced by automated winding machines, thereby reducing the extent to which the structural integrity of the blade relies on bonded joints. Typical filamentary materials found useful are fiberglass, carbon, graphite, Kevlar and boron in matrices of epoxy or polyester. Composite tapes may also be used. With blades of this type the spar, also filament wound, is the main load-carrying element. The use of a filament winding process permits varying the wall thickness and fiber orientation for optimizing strength and stiffness along the blade, providing excellent shear characteristics and the capability of handling large loads.
Most prior art composite blades have steel spars which provide structural support to the blade along its entire length, and permit the use of standard blade retention devices including blade pitch adjusting mechanisms integral with the shank of the blade spar. With blades having filament wound spars, prior art techniques for connecting the blade to the hub are not appropriate due to the spar material differences. With composite spar blades it has been found desirable to separate the blade pitch adjusting mechanism from the blade, the blade pitch forces being transmitted from the pitch adjusting mechanism located in the hub through blade-to-hub adapters to the blade itself. Consequently the blade retention device must be able to hold securely the composite spar while at the same time transmit blade pitch forces from the pitch adjusting mechanism to the blade, and accomplish these results while absorbing the high loads resulting from the large size of the blade.